Wireless Sensor Networks (WSNs) are used in various application areas, including industrial process monitoring and control, environment and habitat monitoring, traffic control, building automation, healthcare applications, etc. In some such applications a powered sensor may be used in a harsh environment, and it is desirable for the sensor to be untethered after deployment for as long as possible. However, most sensors are powered by batteries, and limited battery capacity is a major limitation for deployment of untethered sensor nodes. Finite sensor node lifetime implies finite lifetime of the applications or additional cost and complexity to replace batteries. Energy harvesting is one alternative for increasing the longevity of sensor nodes. If an energy source is periodically available for harvesting, a sensor node may operate for a substantially longer time than if only battery-powered. However, WSNs require very small energy harvesting devices to keep the size of the sensor node and the cost of sensor node deployment to a minimum. Consequently, the amount energy harvested may be relatively small, and device energy use must be correspondingly constrained.